I think i may have had the same problem as you. I had just recently bought my truck '04 and while driving it home for the first time i heard this noise but couldn't pinpoint the location. Loud enough to be annoying. I changed all fluids (oil, diff., trans) to amsoil. I don't think the brand had anything to do with it. i stumbled onto your post and remembered the sound and hadn't heard it since i got new fluids. Maybe i fixed it maybe not, hope that helps you. If i hear it soon i will report back.